This invention relates generally to photomechanical methods of forming pressure sensitive transfer sheets for reproduction of color images and more particularly, provides a method and means for effecting formation of residue-free color images on sheet substrates for artistic and color proofing purposes.
Many photomechanical processes are known for forming color images on selected substrates and then transferring said color images onto selected receptor surfaces by burnishing. Among the common processes are those involving the use of radiation sensitive materials such as photoresists which can be activated by projecting ultraviolet light on the sensitive surface of a photoresist overlay applied to an underlying ink or color carrying material in turn carried by a substrate. The exposed photoresist located at the imaged areas hardens to become a protective layer bonded to the underlying ink layer. The nonexposed areas are unchanged. The unexposed photoresist and the underlying ink areas then are removed by selected solvents in a process which may be referred to as "solvent development". A pressure sensitive adhesive is applied as a layer over the imaged and nonimaged areas as a final step in forming the transfer sheet.
The resulting sheet carrying the adhesive coating is placed image side down on a receptor surface and properly registered. The substrate is burnished at the image portions desired to be transferred while same are in contact with the receptor surface.
The substrate may be provided with a base release coating, usually of a silicone composition, so that when the adhesive is applied over the imaged and nonimaged areas, the adhesive cannot hold to the substrate surface at the nonimaged areas and will be released when burnishing takes place during transfer of the imaged portions. Thus one can state that the adhesive has a greater affinity for the receptor surface than for the siliconed substrate at the burnished areas. The hardened areas defining the image have greater affinity for the adhesive than for the substrate and hence, will adhere to the adhesive at the burnished areas. The underlying ink areas being firmly bonded to the hardened areas (of the imaged photoresist material) also transfers with the adhesive and underlying hardened photoresist areas. The bond formed between the nonimaged substrate areas, the respective intermediate layers and the adhesive coated areas, the nonimaged substrate areas and the respective imaged areas is not as strong as the bond formed between the receptor surface and said adhesively coated areas so that the entire area which has been burnished is transferred to the receptor surface with adhesive residue.
Another problem encountered in prior aforesaid color imaging methods is that they do not use a base release coating but put the release agents in the ink itself which involves the bleeding of the inks into the body of the underlying surface or into the substrate as nonremovable stains. Other problems include the formation of undesirable adhesive halos about the transferred images, the transfer of only portions of the color image, the high cost of the speciality (release) inks, the difficulty in mixing the inks with release materials and unsatisfactory bonding to certain receptor surfaces. Nonuniformity and even pinholes are encountered. Difficulty in transferring fine line images also is experienced with prior methods and image transfer sheet. Additionally, the transferred images often are flat in appearance rather than possessing a desired gloss finish.